1. Field of the Invention
The present invention relates to a power amplifier and more particularly to a microwave power amplifier topology that provides predistortion to precompensate for gain compression and phase expansion of a downstream power amplifier.
2. Description of the Prior Art
Radio frequency and microwave communication systems are known to place ever-increasing demands on the linearity and efficiency of power amplifiers. Unfortunately, conventional power amplifiers operate at maximum efficiency at or near saturation. Thus, in order to accommodate communication signals having varying amplitudes, systems utilizing conventional power amplifiers normally operate at less than peak efficiency for a substantial portion of the time.
In order to solve this problem, so-called Doherty amplifiers have been developed. Doherty amplifiers were first introduced by an inventor having the same name in; “Radio Engineering Handbook” 5th edition, McGraw Hill Book Company, 1959, pp. 18-39, as well as U.S. Pat. No. 2,210,028. The standard topology for a Doherty amplifier includes a carrier amplifier, operated in a Class AB mode and peak amplifier operated in a Class C mode. A quadrature Lange coupler is used at the input so that the carrier amplifier and peak amplifier signals will combine in phase. A quarter wave amplifier is provided at the outputs of the amplifier. In essence, the carrier amplifier operates at a point where the output begins to saturate for maximum linear efficiency. The peak amplifier is used to maintain the linearity of the output signal when the carrier amplifier begins to saturate.
Such Doherty amplifiers have been known to be used in various microwave and RF applications. Examples of such applications are disclosed in U.S. Pat. Nos. 5,420,541; 5,880,633; 5,886,575, 6,097,252 and 6,133,788. Examples of such Doherty amplifiers are also disclosed in “A Fully Integrated Ku-Band Doherty Amplifier MMIC,” by C. F. Campbell, IEEE Microwave and Guided Wave Letters, Vol. 9, No. 3, March 1999, pp. 114-116; “An 18-21 GHz InP DHBT Linear Microwave Doherty Amplifier”, by Kobayashi et al, 2000 IEEE Radio Frequency Integrated Circuits Symposium Digest of Papers, pages 179-182; “A CW 4 Ka-Band Power Amplifier Utilizing MMIC Multichip Technology,” Matsunaga, et al., 1999, GaAs IC Symposium Digest, Monterey, Calif., pp. 153-156, all hereby incorporated by reference.
Known Doherty amplifiers experience gain compression and phase expansion as the input power level is increased. While gain expansion and phase compression can be achieved operating the amplifier in Classes B and C, such operation increases the intermodulation (IM) distortion. Thus, a circuit is needed to compensate for the gain compression and phase expansion as the input power level is increased without increasing the IM distortion.